1. Field of the Invention
The present invention relates to methods and apparatus for focusing a wafer stepper and more particularly to such methods and apparatus in which a focusing reticle is used to project an image of alternating bands of light onto the surface of a wafer.
2. Description of the Related Art
In the fabrication of integrated circuits, a plurality of such circuits are typically formed on a single wafer. When wafer fabrication is complete, the wafer is cut so that each circuit is contained on a single piece of wafer which is known as a die.
The integrated circuits are formed on the wafer using a lithographic process in which the wafer is coated with a layer of photoresist. A machine known as a wafer stepper is used to project the image of a mask onto the photoresist. The mask image defines a pattern which makes up one of the layers in the integrated circuit. Since a plurality of such circuits are formed on a single wafer, the wafer stepper moves the wafer in a predetermined pattern to permit the mask image to be projected onto each area of the wafer upon which an integrated circuit is formed. After the image is projected onto each such area, the wafer is developed and additional processing is performed. The wafer is again coated with photoresist and the wafer stepper is used to project the image of a different mask onto each circuit in the wafer thereby forming the next layer in the integrated circuit.
The wafer stepper includes a light source, a mask holder, a focusing lens and a wafer table for supporting a wafer thereon. In operation, light from a lamp passes through the mask and then through the lens which projects an image of the mask onto the surface of the wafer. The lens and wafer table are arranged so that the focal plane of the lens and the surface of the wafer supported on the table are substantially parallel. The wafer table is moveable in increments as small as 0.2 micron so as to move the table toward and away from the lens focal plane while maintaining the focal plane parallel to the table. This facilitates precision focusing of the lens. Such focusing is necessary to produce lines in the lithographically-created image on the wafer which can be smaller than one micron in width.
One prior art method and apparatus for focusing the lens of such a wafer stepper on a wafer projects the image of a reticle onto the surface of the wafer. The reticle includes alternating opaque and transparent bands which are each of the same with. The image projected thus comprises alternating bands of light and darkness which are substantially equal to one another in width. The reticle includes a series of differnt sized bands having widths ranging from about 0.4 to about 2.0 microns. Each set, regardless of its band width, includes alternating transparent and opaque bands having equal widths.
When using such a reticle to focus a wafer stepper, the stepper is first placed in nominal focus. A wafer supported on the wafer table includes a layer of photoresist thereon. The reticle image is sequentially exposed on a number of different locations on the wafer with the wafer table being vertically moved a preselected distance, e.g., 0.3 micron, between each exposure. Thereafter the wafer is developed and examined through a microscope. Images which are out of focus include bridging or smudging between adjacent lines formed in the photoresist. The image which has the best focus is selected and the wafer table is set to the same position at which the in-focus reticle image was obtained.
Gage repeatability is the variation in measurements obtained when one operator uses the same gage for measuring the same characteristic of the same part. Empirical testing of the prior art reticle described above found poor repeatability for determining which reticle image is best focused. The average range of the wafer table position was 0.4 microns. In other words, the same operator using the same exposed wafer to focus the wafer stepper could set the table anywhere over a 0.4 micron range each time the stepper is focused.
It would be desirable to more accurately focus the wafer stepper.